Solid, ionic conductive elements are known and have been used in hydrogen-oxygen fuel cells, as is well known to those skilled in the fuel cell art. The use of such solid, ionic conductive electrolyte elements in an electrochemical gas sensor has also been demonstrated, however, has not been heretofore proposed or used in such electrochemical gas sensors to solve the problems of electrode flooding in the typical prior art gas sensor, as we presently understand the prior art.
The present invention provides an improved, less expensive and simpler construction for an electrochemical gas sensor as well as a simplified operation without the prior problems of flooding the electrodes caused by the use of wet ionomer membrane or resins during fabrication. The electrochemical gas-sensing cell of the present invention is capable of sensing concentrations of electrochemically active gases in gas mixtures in the parts per billion range. The use of a dry ionomer membrane in the gas sensor fabrication eliminates the problem of flooding of the electrode surface in sensors manufactured utilizing a wet or pre-equilibrated ionomer membrane. The present invention utilizes a solid dry ionomer membrane to manufacture an electrochemical sensor. At a desired time after assembly, the ionomer membrane can be equilibrated with water so that the membrane obtains significant ionic conductivity at room temperatures. By postponing hydration of the ionomer membrane, calibration time of electrochemical sensors is unexpectedly reduced. The dry ionomer membrane can be utilized with film-based techniques, which have been widely investigated in electrochemical sensor microfabrication technology.
Film based techniques in microfabrication technology are known for a wide variety of sensors. Solid-state gas sensors have demonstrated the advantage of being able to operate at elevated temperatures, however they have the disadvantages of slow response and recovery time and a high internal operating temperature. The disadvantages and limitations of the state-of-the-art sensors prevent efficient usage of such sensors in battery-powered instruments.
Nafion®-coated metal oxide pH sensor with sputtered iridium oxide sensing and silver/silver chloride reference electrodes on alumina ceramic substrates are also known in the art. Nafion® has been used as a cation-selective ionomer coating in order to decrease the oxidation-reduction error generally affecting the performance of metal oxide pH electrodes. The use of Nafion® as polymer-electrolyte for a thin-film CO sensor with macro-sized, sputtered Pt sensing and counter electrodes and a smaller, sputtered Au electrode as reference electrode is also known in the art. A 5 wt % n-propyl alcohol solution of Nafion® (DuPont, 1100 EW) is used to form the polymer electrolyte film over the electrodes by casting. The polymer is washed and protonated in aqueous sulfuric acid prior to casting. The reported lifetime of this sensor is reported to be less than one month. During this time, the CO oxidation current decreases steadily down to a few percent of its original value without any period of stable measurement signal. The lifetime of the device may be extended up to three years by laminating the polymer electrolyte layer with a cast perfluorocycloether-polymer film in order to keep the CO permeability coefficient through Nafion® constant; theoretical calculations showed that the drift rate of the signal could be significantly reduced under these conditions.
Nafion® is a copolymer of perfluoro-3,6-dioxa-4-methyl-7octene-sulfonic acid and tetrafluoroethylene (Teflon). Nafion® can be described as having a Teflon backbone with occasional side chains added of another fluorocarbon. The side chain terminates in a sulfonic acid (—SO3H). With the exception of the sulfonic acid group, all of Nafion® is a fluorocarbon polymer. Like most fluoropolymers, it is extremely resistant to chemical attack (corrosion resistant). The sulfonic acid group is immobilized within the bulk fluorocarbon matrix and cannot be removed, but unlike the fluorocarbon matrix the sulfonic acid groups do participate in chemical reactions. The presence of the sulfonic acid adds three important properties to Nafion®: 1) Nation® functions as an acid catalyst due to the strongly acid properties of the sulfonic acid group; 2) Nafion® functions as an ion exchange resin when exposed to solutions; 3) Nafion® very readily absorbs water, from the vapor phase or from the liquid phase. Each sulfonic acid group will absorb up to 13 molecules of water. The sulfonic acid groups form ionic channels through the bulk hydrophobic polymer, and water is very readily transported through these channels. Nafion® functions like a very selective, semi-permeable membrane to water vapor.
The physical properties of Nafion® are similar to other fluoropolymers. It is a translucent plastic, with reasonable flexibility. When used as an ion exchange membrane, it is specified by its manufacturer, DuPont, to operate at temperatures up to 190° C. An unusual property of Nafion® is its propensity to change in physical size. As Nafion® absorbs water, it will swell (increase in size) by up to 22%. When exposed to alcohols it will swell up to 88%.
Table 1 shows readily available types of Nafion® membranes. All measurements were taken with membranes conditioned to 23° C., and 50% Relative Humidity (RH).
Membrane TypeNominal Thickness (mm)Weight Caliper (g/dm2)N-11100250.5N-1120.0511.0N-1135, N-10350.0891.9N-115, N-1050.1272.5N-1170.1833.6
Dry ionomer membranes can also be defined as those ionomer membranes that are hygroscopic. Hygroscopic membranes are those membranes that readily absorb or attract moisture from the air; or membranes having an affinity for moisture. One such example is Nafion® 117 perflourinated membrane manufactured and sold by E. I. du Pont de Nemours and Co. Dry ionomer membranes do not include membranes that have been soaked in any solution such as water, or acidic solution.
The present invention relates to the manufacture of electrochemical sensors using hygroscopic Nafion®, or Nafion® sold in dry sheets to form the ionomer membrane, which has had the unexpected result of facilitating the manufacturing process and the development of sensors with improved start-up times after assembly of the sensor.
Table 2 compares mechanical and electrical properties of dry sheet Nafion® at 50% RH and 23° C. to wet Nafion® soaked in water.
NAFION ® Mechanical and Electrical PropertiesPropertyTypical ValueTest MethodTensile Modulus,MPa (kpsi)50% RH, 23 C. 249 (36)ASTM D 882water soaked, 23 C. 114 (16)water soaked, 64 (9.4)100 C.Tensile Strength(max), MPa (kpsi)50% RH, 23 C. 43 (6.2) in MD, 32 (4.6) in TDASTM D 882water soaked, 23 C. 34 (4.9) in MD, 26 (3.8) in TDwater soaked, 25 (3.6) in MD, 24 (3.5) in TD100 C.Elongation at Break,%50% RH, 23 C. 225 in MD, 310 in TDASTM D 882water soaked, 23 C. 200 in MD, 275 in TDwater soaked, 180 in MD, 240 in TD100 C.Tear Resistance -Initial, g/mm50% RH, 23 C.6000 in MD, TDASTM Dwater soaked, 23 C.3500 in MD, TD10004water soaked,3000 in MD, TD100 C.Tear Resistance -Propagating, g/mm50% RH, 23 C.>100 in MD, >150 in TDASTM D 1922water soaked, 23 C. 92 in MD, 104 in TDwater soaked, 74 in MD, 85 in TD100 C.Density, g/cm3  2.0—Conductivity, S/cm  0.083
Table 2 shows that tear resistance (g/mm) of dry membrane increases with thickness. These values for tear resistance are typical of N-112 0.051 mm membrane.
Where specified in table 2, “MD” means machine direction, and “TD” means transverse direction. Also, conductivity measurements made for 1100 EW membranes utilizing membrane conditioned at 100 C water for 1 hour. The conductivity measurement cell was submersed in 25 C water during experiment, and membrane impedance (real) taken at zero imaginary impedance.
Table 4 compares water uptake from dry Nafion® membrane (dry weight basis) to water soaked Nafion® membrane at 100° C. for 1 hour.
NAFION ® Hydrolytic PropertiesTypicalPropertyValueTest MethodWater Uptake, % water35ASTM D 570Thickness Change, % Increasefrom 50% RH, 23 C. to water soaked, 23 C.10%ASTM D 756from 50% RH, 23 C. to water soaked, 100 C.14%Linear Expansion, % Increasefrom 50% RH, 23 C. to water soaked, 23 C.10%ASTM D 756from 50% RH, 23 C. to water soaked, 100 C.15%
A description of typical state-of-the-art hydrated solid polymer electrolyte or ionomer sensors and sensor cells is described by Kosek et al. U.S. Pat. No. 5,527,446; LaConti and Griffith, U.S. Pat. No. 4,820,386; Shen et al., U.S. Pat. No. 5,573,648; and, Stetter and Pan, U.S. Pat. No. 5,331,310 all of which are herein incorporated by reference. These sensor cells, based on hydrated solid polymer electrolyte or ionomer technology, have several advantages over conventional electrochemical sensor cells. The catalytic electrodes are bonded directly to both sides of a proton conducting solid polymer ionomer membrane providing a stable electrode to electrolyte interface. One side of the electrolyte membrane is flooded with distilled water, making the sensor cell self-humidifying and independent of external humidity. Since no corrosive acids or bases are used in the sensor cell, a lifetime of over 10 years has been demonstrated for solid polymer ionomer sensor cells. Finally, the sensor cells are easy to maintain, and so are ideal for use in remote, unattended environments. Regular addition of water to the reservoir in the sensor housing every several months and monthly calibration checks are the only requirements.
One of the concerns with the state-of-the-art sensors described above is that the signal-to-noise ratio is not conducive to detection of very low concentrations (parts per billion, ppb) of important environmental and biomedical gases and vapors. Response time is relatively slow, and reproducibility between sensors and sensor cells is not high. Also, they are relatively costly.
Recently, miniaturized thick and thin film type sensors have been developed where the solid ionomer membrane acts as a conduit between the gas to be detected (sample gas) and the sensing electrode. The sample gas permeates through the membrane itself where a 3-phase contact area is established. The concern with this configuration is that the solid ionomer membrane water content controls the gas permeation rate as well as proton conductivity. As the humidity increases, the membrane water content increases. This causes an increase in the gas diffusion rate as well as proton conductivity and sensor signal response. The best method of controlling or fixing the water content of the membrane is to have a water reservoir on the back side of the membrane, directly opposite to where the film type electrodes and non-conductive supportive substrate are located, however other configurations positioning the water reservoir on the front side of the membrane are possible. Unfortunately in the back side configuration the back side of the membrane is required to be free of liquid so that the sample gas can diffuse through the membrane to the sensing electrode.
Another concern of the state-of-the-art sensors is flooding of the electrode surfaces caused during the fabrication of the sensors. Flooding causes the formation of liquid droplets on the electrode surface and results in decreased sensor sensitivity after assembly. Electrochemical sensor arrangements where an electrode lies immediately adjacent to a hydrated ionomer membrane are prone to flooding. The propensity to flood is further increased with the thickening of the electrode; hence thick film electrodes are more prone to flooding than thin film electrodes.
The propensity of electrode flooding is further increased by the common use of Nafion® as the ionomer membrane of choice. A perfluorosulfonic acid membrane is defined as a polymer that contains small proportions of sulfonic or carboxylic ion functional groups. Nafion® is typically cleaned extensively by boiling in water to remove impurities. The use of wet Nafion® in the manufacturing process results in the formation of liquid droplets on the electrode; hence sensors are formed with decreased sensitivity.
Typically, sensors with flooded electrodes need to be flushed with dry gas for extended periods greater than 24 hours in order to regain their optimal response rate. One typical embodiment of this invention solves the problems associated with wet Nafion® use by using a dry Nafion® sheet in the production process. This dry Nafion® sheet is obtained in hygroscopic form and has not been boiled, soaked in any liquid, or otherwise treated (i.e. equilibrated in an acidic solution).
The best method for hydrating the ionomer membrane such as Nafion® would be to have a water reservoir located adjacent to the membrane, and opposite to where the film type electrodes are located. These reservoirs can contain a water seal, which may be broken anytime after assembly in order to release water and hydrate the ionomer membrane. Providing an orifice in the sensor housing with a cap enables refillable reservoirs.
Another problem associated with the use of wet Nafion® in electrochemical sensors is that wet parts are difficult to work with. Therefore, making electrochemical sensors with dry Nafion® decreases the difficulty of handling materials during the manufacturing process.
Yet another problem associated with the use of wet Nafion® in the manufacturing of electrochemical sensors is that the wet parts may result in sensors with a varying amount of sensitivity from one another. Hence, using dry Nafion® provides a means of obtaining more uniform results in sensor reproducibility.
The present invention overcomes the limitations of the state-of-the-art in miniaturized electrochemical sensors stated above by uniquely combining a dry ionomer membrane configuration with a thick or thin film type electrode on a non-conductive supportive substrate. The substrate may have diffusion openings or holes having a known area, which permit easy access of the sample gas to a sensing electrode contact area. The sensor configuration provides a three phase contact area that serves as an interface for the ionomer membrane, the electrodes, and the gas being detected. This design utilizes the precision of solid-state device fabrication techniques to yield inexpensive, low maintenance, highly sensitive, rapidly responsive, and reproducible sensor devices for environmental, industrial, and biomedical monitoring.